1. Field of the Invention
This invention relates to thermoplastic adhesive and sealing organic resin compositions which, on application of heat, preferably in an accelerated manner, crosslink to give a thermoset bond or seal.
The invention also relates to a process for forming a crosslinked bond or seal.
2. Description of the Prior Art
The concept of thermosetting or crosslinking resin adhesives is known in the art. Many resin adhesives which undergo an irreversible chemical and physical change and become substantially insoluble are well known. Thermosetting adhesives comprising both condensation polymers and addition polymers are also known and examples include the urea-formaldehyde, phenol-formaldehyde and melamine-formaldehyde adhesives; epoxy, unsaturated polyester and polyurethane adhesives. U.S. Pat. No. 4,122,073 teaches thermosetting resin obtained from polyisocyanates, polyanhydrides and polyepoxides. Crosslinking in these patents is achieved by reaction with available sites in the base polymers. U.S. Pat. No. 4,137,364 teaches crosslinking of an ethylene/vinyl acetate/vinyl alcohol terpolymer using isophthaloyl biscaprolactam or vinyl triethoxy silane whereby crosslinking is achieved before heat activation with additional crosslinking induced by heat after application of the adhesive. U.S. Pat. No. 4,116,937 teaches a further method of thermal crosslinking by the use of polyamino bis-maleimide class of flexible polyimides, which compounds can be hot melt extruded up to 150.degree. C. and undergo crosslinking at elevated temperatures thereabove. In these latter two patents, thermocrosslinking is also achieved by reactions of the particular crosslinking agent with available sites of the base polymers.
U.S. Pat. No. 4,352,723 is directed to dual UV and heat activated compositions comprising (1) a liquid, ethylenically unsaturated monomer, oligomer or prepolymer of the formula: ##STR2## wherein R is H or CH.sub.3, R.sub.1 is an organic moiety and n is 2 to 4, hereinafter referred to as the ethylenically unsaturated compound, (2) a photoinitiator, and (3) a substituted or unsubstituted pinacol. The exposure to UV radiation and heat in seriation or simultaneously results in a cured solid product. U.S. Pat. No. 4,348,515 relates to a heat curing process and to heat activated compositions complrising (1) an ethylenically unsaturated compound containing at least two carbon-to-carbon double bonds and (2) a thiuram. However, as is well known, these acrylate materials set out in these latter two patents when cured are brittle and thus have poor flexibility. U.S. Pat. No. 4,374,963 relates to a heat curable composition comprising (a) a liquid, ethylenically unsaturated monomer, oligomer or prepolymer of the formula: ##STR3## wherein R is H or CH.sub.3, R.sub.1 is an organic moiety and n is at least 2, (b) an epoxy resin containing at least two ##STR4## groups, and (c) a thermal initiator member of the group consisting of (1) a substituted or unsubstituted diaryliodonium salt in combination with a free radical initiator, (2) a BF.sub.3 adduct, (3) A BF.sub.3 adduct in combination with a free radical initiator, and (4) dicyandiamide in combination with a free radical initiator. This system has improved flexibility but requires coinitiators to cure the different curable materials.
In substantially all of these thermosetting adhesives bond formation is dependent on the chemical crosslinking reaction which in most cases is accelerated by means of heat to obtain the bond within a reasonable period of time. Further, in many cases, two or more components must be admixed just prior to the preparation of the bond. This necessitates a fast application since the crosslinking reaction begins immediately upon admixture and is irreversible. Thus, there has been a desire for a one part thermosetting adhesive which can be applied and thereafter triggered to cure on command.
Methods of achieving delayed tack are known in the art. See U.S. Pat. Nos. 2,653,880, 2,653,881 and 4,059,715 which teach the employment of thermoplastic polymers containing slowly crystallizing segments.
On the other hand, thermoplastic adhesives, which are used in the form of solutions, dispersions or solids, usually bond by purely physical means. Probably the most important means of applying thermoplastic adhesives is the hot melt method wherein bond formation occurs when the polymer melt solidifies in position between adherends. The bonds obtained by this method reach their final strength faster than those obtained from solution type adhesives. Obviously, the thermal stability of the thermoplastic resin determines its potential usefulness as a hot melt adhesive. In order for the thermoplastic to be used as a hot melt, it must also have a low melt viscosity, thus permitting application of the adhesive to the adherends at acceptable rates. Usually this means the polymer must have a low molecular weight. However, many thermoplastic materials cannot be employed as hot melts because they do not have sufficient cohesive strength at the low molecular weights required for application to a substrate. For example, the low molecular weight polyolefins, especially low molecular weight, low density polyethylene, are widely used in hot melt adhesives for sealing corrugated cartons, multi-wall bag seaming and the like, but they do not have sufficient strength to be used in structural applications such as plywood manufacture. Further, they do not have sufficient heat resistance to be used for bonding components which are intermittently exposed to elevated temperatures such as under the hood automotive applications. That is, thermoplastic adhesives cannot be employed where the adhesive in situ is reexposed to elevated temperatures which will cause the adhesive to sag thereby allowing the bond to break.
In the prior art there are many two-part materials which are cured in situ at elevated temperature, e g., epoxy and urethane resins. The curing times, however, are relatively long, thereby precluding on-line production in a continuous operation. The curing time can be substantially reduced by heating, but such methods are rarely used due to the fact that external heating also causes substrate or adherends to be heated. In the case of heat sensitive substrates and adherends, e. g., thermoplastics, it can cause damage or distortion thereof.